The advancement of portable electronics and the continual integration of functionality into a single all-encompassing device has created an increased demand on energy supply. The incumbent Li ion battery is not projected to sufficiently accommodate this growing demand. An attractive alternative for devices operating in the <100 W range is the direct methanol fuel cell (DMFC). The DMFC could potentially bridge the gap in performance, as methanol has a high energy density (4820 Wh L−1), can be continuously operated through the replacement of a fuel cartridge, and can be easily handled through existing infrastructure.
The DMFC can be operated under a passive or active configuration. The target application largely determines which one is used. For higher power devices (>10 W) an active system is preferred because higher performances can be achieved through the careful control of operating conditions. A typical active DMFC system include balance of plant components to control the operating conditions. A series of sensors, pumps and fluid control systems manage the temperature, humidification and fuel/oxidant stoichiometry of the fuel cell. Additionally the convective nature of the feed streams allow for improved mass transfer and the removal of waste products such as carbon dioxide and water. Although higher power outputs can be achieved, active systems tend to be larger, more complex, and suffer from parasitic power losses due to auxiliary components and electronics. These characteristics limit their use in smaller portable electronic devices in the subwatt to 10 W range.
In contrast with active systems, a passive DMFC system is simple, compact and does not include auxiliary control components. These characteristics are attractive for the integration into small portable devices. In a passive system the fuel and oxidant are supplied through non-parasitic power processes such as capillary action, diffusion and natural convection. The power output however tends to be lower as a result of mass transport limitations with respect to waste product removal of carbon dioxide at the anode and water management at the cathode.
In a DMFC, an aqueous methanol fuel and an oxidant, typically air, are generally used. The electrochemical reactions for this type of fuel cell at ambient temperature and pressure (25 C, 1 atm) are shown in equations 1-3.
Anode Half-cell Reaction:CH3OH(l)+H2O(l)→CO2(g)+6H++6e− Ea°=−0.016V  (1)Cathode Half-Cell Reaction:3/2O2(g)+6H++6e−→3H2O(l) Ec°=1.229V  (2)Overall Reaction:CH3OH(l)+3/2O2(g)→2H2O(l)+CO2(g) E°=1.213V  (3)
The DMFC is an example of direct liquid fuel cells that use liquid fuels directly as the fuel, and a number of architectures for such cells are known in the art. At the core of a conventional DMFC is the membrane electrode assembly (MEA). It consists of a solid polymer electrolyte membrane (PEM) compressed between an anode and cathode diffusion electrode. The electrodes are typically made from a Teflon® coated carbon cloth, paper or felt with a carbon supported catalyst layer applied to a single side. Nafion® is commonly used as an electrolyte due to its high ionic conductivity and good thermal and mechanical stability. A common difficulty for conventional fuel cell technologies is the ability to manage variable power demand conditions, for example as may occur in vehicles or in electronics powered by fuel cells in which changes in power demand are frequent. Fuel cells are typically configured for optimal power output under specific conditions, and when these conditions are changed, the fuel cell must then operate under sub-optimal conditions. Efforts to address this issue with fuel cells have generally focused on the so-called balance of plant (BOP) aspects of the fuel cell system in which various methods have been devised to alter the power output of the system. For example, in active DMFCs, a common practice is to control the concentration of the methanol fuel via a series of sensors, pumps and valves that manipulate the concentration of the fuel being fed into the fuel cell to compensate for the variable power demands of the device being powered. This not only leads to increases in system complexity and cost, but also to the fuel cell operating under suboptimal conditions, frequently resulting in lowered efficiency, performance and durability. For passive systems, the issue is more serious as they do not contain additional BOP components to control the fuel concentration or stoichiometry at different power levels. Similar issues are faced in hydrogen fuel cells.